Stenolaemate Stories
The stenolaemate bryozoans quickly radiated in the early
Paleozoic and
are very characteristic fossils of Paleozoic rocks, sometimes making
substantial contributions to the formation of reefs, calcareous shales, and
limestones. They included forms with robust skeletons, such as the
trepostome Hallopora pictured above; such forms were common in
shallow-water habitats that today are dominated by
corals.
There were also forms with delicate,
branching fanlike skeletons such as the fenestrates pictured below (from
the Mississippian of Domodedovo, near Moscow, Russia). With the exception of
one order of stenolaemates, the Tubuliporata or Cyclostomata, all of these
Paleozoic bryozoan lineages were severely impacted in the Permian extinction:
cryptostomates disappeared at the end of the Permian (245 million years ago),
while a few other lineages lingered until the end of the Triassic, about 210
million years ago. Tubuliporate bryozoans have survived to this day, and
in fact underwent a remarkable radiation in the Cretaceous, but are no
longer dominant today.

Gymnolaemate Grandeur
Uncalcified gymnolaemates are known as fossils from the Late Ordovician
on, almost exclusively as distinctive borings in carbonate substrates such
as shells. Non-boring, non-calcified gymnolaemate bryozoans are extremely
rare as fossils and known from the
Jurassic and Cretaceous only.
Calcareous gymnolaemates did not appear in the oceans until the
Cretaceous, during which time they diversified rapidly from a very few
species in the early Cretaceous. By the end of
the Cretaceous, there were over 100 genera of gymnolaemates. They continued
to diversify in the Cenozoic: today there are over 1000 genera, comprising
the bulk of bryozoan diversity in today's seas.